1. Field of the Invention
This invention relates in general to switching systems for light waveguides and in particular to a novel switching system which prevents undesired connections when voltage outages occur in a switching matrix network formed from optical binary couplers.
2. Description of the Related Art
Recent developments in telecommunications technology have resulted in light waveguide (LWG) communication system in which not only the subscriber lines extending between the exchange and the subscriber stations are formed by light waveguides as disclosed in U.S. Pat. No. 3,980,831 German Pat. No. 2 421 002), but also the through-connections in the exchange can be produced using light guiding integrated optics (IO) 2.times.2 switch elements for example, in the form of controllable electro-optical directional couplers as described in the paper ISS'84 Conference Papers 41A4, FIGS. 3, 5, 6 and 8; and 1986 International Zurich Seminar Conference Papers C7, FIGS. 1, 2 and 4.
In a controllable electro-optical directional coupler, two strip waveguides which are narrow thin strips produced in a substrate by driving in, for example, titanium into lithium niobate and which have a higher optical refractive index than the substrate are formed in extremely close proximity to each other at a typical distance of about 5 .mu.m in a coupling region of a defined length so that the optical fields overlap and light energy can be coupled from one strip waveguide onto the other strip waveguide. Control electrodes which receive an electrical control signal which influences the cross-over are located in the coupling region next to and/or between the strip waveguides and when an adequate corresponding control voltage exists, no crossover occurs. In the absence of a control voltage, a complete cross-over can occur as is discussed in the article ntz 39 1986) 12, Pages 828 . . . 830 and FIGS. 3c and 3d; Telcom Report 10(1987) 2, Pages 90 . . . 98, FIG. 8.
Since 2.times.2switching elements which are only capable of two switching conditions which are uncrossed through-connection and the cross through-connection are also referred to as binary couplers as shown in U.S. Pat. No. 4,351,985 (German Pat. No. 2 036 176), (in communication technology) controllable electro-optical directional couplers utilized as 2.times.2 switching elements can also be referred to as optical binary couplers.
Larger switching networks can also be formed with binary couplers as shown in U.S. Pat. No. 4,351,985 (German Pat. No. 2 036 176); ISS`84 Conference Papers, Loc. Cit., 1986 International Zurich Seminar Conference Papers Loc. Cit.; and, for example, an optical 4.times.4 switching network can be realized with at least five optical binary couplers contained in an integrated optics IO module and arranged in three stages as shown in the publication Electronics Week, March 18, 1985, Pages 55 . . . 58, FIG. 7.
The fact that when using switching networks formed with binary couplers, the individual binary couplers are always in one or the other through condition in a switching matrix network formed with optical binary couplers all binary couplers are in the condition of cross through-connection in the absence of all control voltages as, for example, due to the outage of the supply results in a problem when it is desired to avoid any undesired connections.
A list of art which can serve as background for the present invention comprises European Patent Application No. 0 146 275, the article High Speed Wide Band Digital Switching and Communication Utilizing Guided Waveguides by Lars Thylen, Pages 113 through 116. ISS'84 Florence 7-11, May 1984, Session 41a Paper, Pages 1-7, Electronics Week, March 18, 1985, Pages 55-57, German Patent No. 2 036 176, German DE 30 25 083, German Patent 582,206, German 1 115 775; publication Forschung-Entwicklung ntz Bd 39 (1986), Pages 828-830, Telcom Report 10 1987, Trends Pages 90-98, German DE 30 25 083, German DE 32 30 251, German DE 32 35 266, German DE 34 23 221, German 36 26 060.